Processes for the preparation of hydrocarbons, in particular an at least partly liquid hydrocarbonaceous product, by the catalytic reaction of carbon monoxide with hydrogen (a mixture often referred to as "synthesis gas") in a reaction zone containing the catalyst bed are well known. This reaction is highly exothermic and cooling means are used in a reactor for carrying out such process to remove heat from the reacton zone. Additionally, such a reactor is usually provided with means to recycle gas through the catalyst bed for equalizing the temperature of the catalyst bed, thus avoiding the formation of hydrocarbonaceous deposits on the catalyst.
When a reactor comprising a fixed bed of the aforementioned type is to be shutdown the supply of carbon monoxide and hydrogen is interrupted. In the reactor space above the catalyst bed, usually a dome-like space, a large amount of reactant gas mixture is present, which will pass through the catalyst bed at a relatively low velocity. At such low velocities, the reaction heat may be insufficiently removed and hot spots may be formed in the catalyst bed. These hot spots result in a deterioration of the performance of the catalyst.
The possibility for the formation of hot spots in the catalyst bed of fixed bed reactors in general and resultant damage to the catalyst is known in the art. Thus, in UK Patent Application No. 2,169,309 a process for the shutdown of a reactor in use in the conversion of oxygenates, such as alcohols, into gasoline is disclosed. In the process of this UK application, when the circulation of recycle gas to the reactor falters, automatic valves terminate the feed of oxygenates to the reactor and then inject inert gases into the reaction zone to quench the reactor.
According to UK Patent Application No. 2,223,237 the above-mentioned problem of overheating the catalyst may be overcome by carrying out the following steps:
(i) interrupting the feed of synthesis gas; PA0 (ii) depressurizing the reactor downstream of the catalyst, and providing the reactor upstream of the catalyst with inert gas and/or hydrogen, preferably hydrogen; and PA0 (iii) cooling the catalyst to ambient conditions.
In one embodiment the dome-like space above the catalyst bed is filled with packing bodies, the bodies containing hydrogen releasable therefrom when the pressure in the reactor falls below the working pressure. The inert bodies may comprises an interfacial membrane permeable to hydrogen and impermeable to carbon monoxide, or the inert bodies may comprise material which absorbs hydrogen under reaction conditions and desorbs under shutdown conditions.
Surprisingly, it has now been found that the decline in performance of a hydrocarbon synthesis catalyst in a fixed bed reactor, occurring as a result of the shutdown of the reactor during operation of the synthesis process, can be significantly reduced by maintaining a reservoir of hydrogen-rich gas at a pressure above the pressure of the feed gas and arranged such that hydrogen-rich gas can flow from the reservoir into the fixed bed during operation. By such a process, it is possible to protect the catalyst during shutdown by rapidly quenching the reaction with a hydrogen-rich atmosphere.